Weld electrode for attractive weld appearance

ABSTRACT

A welding electrode for use in resistance spot welding an assembly of overlying metal workpieces that includes an aluminum alloy workpiece is disclosed. The welding electrode includes a body, a convex weld face at one end of the body, and ringed protrusions that project outwardly from the convex weld face. The ringed protrusions are positioned to make contact with, and indent into, a surface of the aluminum alloy workpiece when the convex weld face is pressed against the aluminum alloy workpiece during a spot welding event. When brought into contact with the surface of the aluminum alloy workpiece, the ringed protrusions disrupt the oxide film present on the aluminum alloy workpiece surface, which improves the spot welding process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/251,635,filed Oct. 15, 2008, which is continuation-in-part of application Ser.No. 11/536,001, filed Sep. 28, 2006, now U.S. Pat. No. 8,436,269. Thetext and drawings of application Ser. Nos. 11/536,001 and 12/251,635 areincorporated herein by reference in their entireties.

TECHNICAL FIELD

This invention pertains to electrodes for the formation of electricalresistance spot welds between two or more sheet metal layers. Morespecifically this invention pertains to welding electrodes for formingof high quality weld nuggets between the metal layers and forsimultaneously forming a pleasing pattern or image at the position wherethe welding electrode contacts the surface.

BACKGROUND OF THE INVENTION

The above identified parent application describes welding electrodeswith a round weld face for contact with a metal workpiece in anelectrical resistance welding operation. The weld face includesconcentric rings of ridges and/or grooves extending radially from thecenter of the weld face; the ridges extending axially upwardly from theface and the grooves extending inwardly into the face. When the face ofthe electrode is pressed into contact with a surface of the metalworkpiece for delivery of a welding current, these shaped features onthe face of the electrode penetrated surface oxides or otherconductivity barriers to facilitate passage of a welding current.

Current automotive vehicle manufacturing operations include, forexample, the joining of two sheet metal layers by spot welding. Vehiclebody panels such as doors, hoods, deck lids and liftgates are oftenassembled by joining inner and outer panels stamped from sheet metal ofsuitable metal alloys. Ferrous or aluminum alloys are often used. Thethickness of each sheet metal layer may vary from less than onemillimeter to more than four millimeters. Electrical resistance spotwelding is often used to join such inner and outer panels or other metalparts. For example, an edge of an outer panel sheet may be folded over acomplementary edge of an inner panel sheet in an assembly of the panelsin which the hem(s) is at the periphery of the sheets. The panelassembly is positioned for welding in areas removed from the hem joint.Axially aligned and opposing electrodes are pressed toward each otheragainst opposite sides of the panel assembly. A momentary weldingcurrent is passed between the electrodes through the layers of metal toform a spot weld. The spot weld is characterized by a momentarily fusedpool of metal and a re-solidified weld nugget at the interface of thecontacting sheets. The electrodes are retracted and moved to anotherweld site. The welding electrodes of the parent application withconcentric features on their welding faces form high quality welds inmetal workpieces.

In some applications spot weld sites may be visible in the surface of afinished product such as an automotive vehicle body panel or member.Several types of joints are used to join sheet metal panels in areasthat may be visible to customers. These areas include joints formed onvehicle body closure members such as hoods, decklids, liftgates, anddoors. Visible joints in the vehicle structure also include those in theengine compartment and trunk. It has been observed that the weldingelectrodes of the parent application not only form strong durable spotwelds in such welded articles, but the circular ridges or grooves leavea faithful impression on a visible surface of a spot weld site. Theclear visible ring pattern in the surface of the welded assembly isfound to be much more pleasing to a consumer than the traditionallyformed welding electrode impression. Such a pleasing weld appearance canalso influence the perceived quality of the assembly, that is, theappearance details alone can influence the consumer's perception as towhether a product will fulfill his or her expectations. And where aprogressive sequence of such weld sites is visible, the suggestion ofhigh quality is reinforced.

Resistance spot weld appearance can suffer from several undesirablefeatures when using current, conventional spot weld electrodes. Forexample, a domed electrode with a flat welding face machined in itscenter is widely used for spot welding steel. It tends to leave a sharpimprint on the sheet surface that can have excessive indentationespecially if the electrode engages the sheet surface in an off-normalorientation when forming a spot weld. The electrode shape is also proneto causing sheet deformation around the weld that is unattractive. Metalexpulsion from excessive heat and off-normal orientation can lead towhiskers or fingers of metal protruding from the sheet surface which isundesirable.

Such welding surface issues have not arisen when using the electrodes ofthe parent application. So it is an object of this invention to provideadditional welding electrode face features that assure the formation ofhigh strength or structural spot welds in workpieces, such as steel andaluminum sheet metal workpieces, while leaving behind clear visibleindicia in a weld surface that provides an attractive appearance.

SUMMARY OF THE INVENTION

This invention is a method to improve resistance spot weld appearance byintroducing visible features on the joint's visible surface. Theappearance is controlled by using an electrode shape that deliberatelyimparts a selected design on (or into) the sheet surface by takingadvantage of the heat and pressure used during the spot weldingoperation. Shaped features are incorporated in at least one of theopposing electrode welding faces that contacts at least one of the outersheet metal surfaces. These shaped features on the electrode facecontribute to the formation of a suitable spot weld and leave animpressed image in the weld surface. The image may be formed usingeither a protrusion extending from the weld face surface, an intrusioninto the weld face surface, or a combination of protrusions andintrusions. The image may be in the form of lines, geometric figures,alphanumeric symbols, or other recognizable and distinguishable images,such as a trademarked logo. The formation of such a discernable imageprovides an attractive appearance, but is not necessarily indicative ofthe underlying weld strength.

In the making of resistance spot welds between overlying sheet layers aweld nugget is formed at the interface of the sheets. The diameter ofthe weld nugget is typically smaller than the diameter of the weld faceof the electrode so the image on the sheet surface at the weld site isnot limited to the size or precise location of the weld nugget.

Formation of the selected design during the welding process may differsomewhat depending on whether the welded material is an aluminum alloyor steel material. For the case of aluminum alloy material, theprotrusion/intrusion on the electrode face forms the design in thematerial surface by reforming the aluminum alloy surface under theelevated temperature and pressure that is produced during the spotwelding process. Aluminum alloys are good conductors of heat andelectrical current. These attributes produce a softened zone around theweld nugget and at the sheet surface which can be deformed into thedesired design. Since the softened zone extends well beyond the formednugget, the design can also extend well beyond the nugget perimeter.

Formation of the selected design in steel materials may be somewhatdifferent. The higher thermal and electrical resistivity of steel, alongwith its high melting point, makes it somewhat more difficult to form adesign in the sheet surface with commonly used copper electrodematerials. However, nearly all steel materials used within automotivevehicle bodies are coated with a layer of zinc or zinc-iron. During thetraditional spot welding process, this zinc layer is typically meltedearly in the welding process and displaced from beneath the electrode byhydraulic pressure. If an electrode is used with a design in thesurface, the molten zinc may be trapped within the design so that it isnot fully displaced from beneath the welding electrode. Provided thatthe electrode temperature does not exceed the boiling point of zinc orzinc-iron, once weld current is stopped, the molten zinc or zinc-ironre-solidifies taking the shape of the electrode surface. The design isthen incorporated by a combination of sheet deformation and “re-casting”of the surface zinc. Since the design is partially formed by recastingzinc, which requires a low temperature to melt, the design can extendwell beyond the nugget perimeter where sheet metal is colder and moredifficult to form.

In each welding application, the electrode face is sized and shaped toform a good weld at the interfacial surfaces of contacting sheetsurfaces, i.e., the faying interface. And the electrode faces are shapedto leave a corresponding pattern of intrusions and/or protrusions in thewelded surface that are intended to suggest to a viewer that the imagein the surface indicates that a predetermined and suitable high qualityweld has been formed. In order to produce an observable feature in thewelded surface(s) it is preferred that protrusions or intrusions on theelectrode faces have a minimum height/depth of about ten microns. As isdescribed below in this specification the protrusions/intrusions may belarger depending on the thicknesses of the workpieces to be welded.

Accordingly, one aspect of the present invention is in the design of theface of an electrical resistance welding electrode to affect good weldsand leave a desired surface image. And another aspect of the inventionis in the method of using such electrodes to form spot welds in metalworkpieces, particularly in surfaces of workpieces that are visible to auser of the welded article.

Other objects and advantages of the invention will be apparent from adescription of illustrative embodiments which follows in thisspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation, schematic view, partly in cross-section, ofthe weld face portions of a pair of opposing, axially aligned resistancewelding electrodes about to engage portions of two overlapping aluminumalloy sheets to form an electrical resistance spot weld. The electrodefaces have protrusions for forming a pattern of grooves in the engagedsurfaces of the aluminum sheets.

FIG. 1B shows the electrodes in a withdrawn position after they haveengaged the aluminum sheets and formed a weld nugget at the interface ofthe sheet surfaces. The outer surfaces of the aluminum sheets aredeformed by the protrusions on the electrode faces.

FIG. 2A is a side elevation, schematic view, partly in cross-section, ofthe weld face portions of a pair of opposing, axially aligned resistancewelding electrodes about to engage portions of two overlappingzinc-coated steel alloy sheets to form a resistance spot weld. Theelectrode faces have protrusions for forming grooves in the engagedsurfaces of the zinc-coated steel sheets.

FIG. 2B shows the electrodes engaging and penetrating through the zinccoating on the steel sheets and forming a weld nugget at the interfaceof the sheet surfaces. The outer surfaces of the steel sheets areslightly deformed by the protrusions on the electrode faces.

FIG. 3 illustrates in cross-section an unsuitable protrusion electrodeface shape with an entry angle such that the protrusion could becomejoined to a sheet surface. In this illustration the sheet surface is analuminum alloy sheet surface.

FIG. 4 illustrates in cross-section an unsuitable intrusion electrodeface shape with an entry angle such that the intrusion could becomejoined to extruded material from a sheet surface. In this illustrationthe sheet surface is a zinc-coated steel alloy sheet surface.

FIG. 5 illustrates a surface of a workpiece in which the letters “GM”have been formed at weld sites by intrusions in the weld face of anelectrode.

FIG. 6 illustrates a protrusion or intrusion design for a weld face forforming the image of a snow flake or star in a welded surface.

DESCRIPTION OF PREFERRED EMBODIMENTS

One or more protrusion and/or intrusion elements are conceived,determined, and formed on the weld face of an electrical resistance spotweld electrode. These elements serve to suitably engage a sheet surfaceto be welded to form a suitable spot weld nugget between the contactingsheet surfaces. But the elements also are shaped to provide anattractive appearance when viewed by an observer.

Accordingly, the protrusions and/or intrusions must meet a combinationof requirements. First, excessive indentation of the sheet material isto be avoided. Excessive indentation or metal thinning may weaken theweld and/or surrounding sheet metal. For a weld consisting of two ormore sheets, indentation occurs on the outer surfaces of the twooutermost sheets. The amount of indentation is typically more importantfor the thinner of the two sheets, since any given level of indentationmakes up a larger portion of the thickness of the thinner sheet. In allcases of aluminum and steel welding, the indentation in the outer twosheet surfaces should not be greater than 50% of the sheet thickness.Preferably for aluminum spot welding, since aluminum is more notchsensitive than steel, the amount of indentation should not exceed 20% ofthe sheet thickness. In both cases, a preferred level of indentationwould be about 15% or less of the sheet thickness since this would havea minimal effect on weld durability. Even a 1% indentation of a typicalsheet thickness can leave a pattern that is visible to the human eye.

FIG. 1A is a fragmentary schematic illustration of the opposingelectrical resistance spot weld electrodes 10, 12 poised for engagementwith an assembly of overlapping aluminum sheets 14, 16. Groupings ofthree concentric circular protrusions 18 on the face 22 of electrode 10and three like concentric circular protrusions 20 on the face 24 ofelectrode 12 have been machined on the spherically curved welding faces22, 24.

Electrodes for resistance spot welding are typically formed of copper orlow resistivity copper alloys. They typically have a round body with ashank portion that is secured in a weld gun mounted on a robot or otherwelding apparatus for conducting a suitable welding current to theelectrodes and for locating them in opposing axial alignment againstopposite outer faces of a sheet metal assembly in which one or morewelds are to be formed. As illustrated in FIG. 1A and several of theother drawing figures, the end of the electrodes 10, 12 may be tapered,for example in the shape of a truncated cone or a truncated sphere, to aflat or rounded welding face. In the illustrative embodiments of thisinvention the welding faces (22, 24 in FIGS. 1A and 1B) are sphericallyrounded, suitably with a radius of for example about 20 mm to about 50mm, for easier engagement with a less-then-perfectly aligned workpiecesurface.

The protrusions may be in the configuration of concentric circles,concentric squares or other geometric shapes. The protrusions may beparallel lines, or alphanumeric letters or numerals, or other meaningfulone dimensional or two dimensional indices. In the FIGS. 1A and 1Bembodiment of the invention, the protrusions 18, 20 are in the form ofthree concentric circles that extend outwardly from the rounded face 22,24 of each electrode 10, 12. But in some embodiments it may be desiredto employ the protrusions only on an electrode face that engages asurface of a workpiece that may be visible to a user of a finishedwelded article. The sides of the protrusions 18, 20 are preferablytapered toward the end (as illustrated in FIG. 1A and FIG. 1B) so thatthey may enter into a softened aluminum alloy surface without bonding toit (as is described in more detail below).

In FIG. 1B the electrode faces have engaged the outer surfaces of theoverlying aluminum alloy sheet workpieces to form a spot weld and thenbeen withdrawn. In forming the weld, a welding current was passedbetween the electrodes to heat a generally cylindrical path through theoverlying sheets. A molten metal pool is momentarily formed at theinterface of the sheets 14, 16 which loses heat to the surrounding metaland re-solidifies as a weld nugget 26 that contains metal from bothsheets 14, 16 and joins them at the welded spot. The rounded electrodefaces 22, 24 with their respective protrusions 18, 20 have deformed themomentarily heat softened outer aluminum sheet surfaces to form andleave attractive patterns 28, 30 of three concentric rings embossed inthe outer surfaces of the now spot welded workpieces 14, 16.

FIGS. 2A and 2B illustrate the use of like or similar opposing weldingelectrodes 110, 112 to form a spot weld in an assembly of overlyingzinc-coated steel sheet workpieces 114, 116 (with zinc coatings 115,117). FIG. 2A shows the axially aligned and opposing electrodes 110, 112with their rounded faces 122, 124 and tapered and rounded protrusions118, 120 ready to engage a predetermined welding site on the assembledsheets 114, 116. In FIG. 2B the electrodes 110, 112 are shown inengagement with the welding surfaces and formed the weld nugget 126between the contacting sheets 114, 116. In this illustration there hasbeen less deformation of the steel sheets. But the zinc coating layers115, 117 (which may be several micrometers thick) have been deformed andrecast in the moderately deformed steel surfaces to leave embossedpatterns in the respective outer surfaces of workpieces 114, 116. As inthe embodiment illustrated in FIG. 1B, the embossed patterns willsubstantially be the negative shapes of protrusions 118, 120.

A second requirement of the protrusion/intrusions is that they not causeexcessive sticking between the electrode and sheet stack-up. Excessivesticking can cause serious problems in production environments. FIGS. 3and 4 illustrate examples where the protrusions and intrusions on anelectrode welding face may cause the electrode to become joined to thedeformed sheet surface. FIG. 3 illustrates spot weld electrode 210 withwelding face 222 and its angled protrusions 218 engaging and penetratinga surface of aluminum sheet workpiece 214. Protrusions 218 are nottapered and engage the surface of aluminum sheet 218 at an angledisplaced from a vertical angle. Similarly, in FIG. 4, spot weldelectrode 310 with welding face 322 and its angled intrusions 318engaging a surface of a steel sheet 314 with its zinc coating layer 315.In this example workpiece metal enters the intrusions 318 machined inthe weld face 322 of electrode 310. While in spot welding operations twoelectrodes and welding surfaces are typically involved, in both FIGS. 3and 4, a single welding electrode and sheet are shown to illustrate theangles of engagement of the respective protrusions or intrusions with aworkpiece surface.

The shape of the protrusions (e.g., 218 in FIG. 3) or intrusions (e.g.,318 in FIG. 4) produces contact angles with the workpieces 214 and 314,315 that tend to lock the electrode face to the workpiece surface. Inboth cases the angle between the exterior surface of the sidewalls ofthe protrusion/intrusion and the plane of the sheet surface must becontrolled such that 1) protrusions do not become irreversibly imbeddedin the sheet surface and 2) intrusions do not result in the entrapmentof deformed sheet metal and/or molten zinc. To accomplish this, theangle between any protrusion sidewall and the sheet surface must be lessthan 90 degrees while the angle between any intrusion sidewall and thesheet surface must also be less than 90 degrees. This is illustrated inFIGS. 3 and 4. In FIG. 3 protrusion surfaces 240 have preferred contactangles and protrusion surfaces 242 have non-preferred contact angles. InFIG. 4 intrusion surfaces 340 have preferred contact angles andintrusion surfaces 342 have non-preferred contact angles. In thenon-preferred angles it is seen that when an electrode is withdrawn thehot soft metal would tend to pulled against the protrusion or intrusionof the electrode face.

The desired (or preferred) angles should not be too close toperpendicular to the workpiece surface, 90 degrees, (e.g. <87 degrees)since walls vertical to the sheet surface might promote sticking. Inaddition, sharp corners in the protrusion/intrusions may promotesticking, rounded corners would be preferred.

A third requirement is that the electrode weld face is shaped withprotrusion and/or intrusion features to allow it to perform its primaryfunction, i.e., produce structural welds in the sheet material.Excessively deep intrusions and especially protrusions on the electrodesurface may possibly have deleterious effects on weld formation.Electrode weld faces, and particularly those designed to spot weldaluminum, are designed to control the current density during the weldingprocess. Radiused electrode weld faces are used to provide a highinitial current density by providing a small contact area between thesheet surface and electrode. Compromising this area by placingintrusions on the weld face should have a small effect as long as mostof the contacting area is left unmodified. No more than 50% of thecontacting area should be modified by intrusions; preferably this shouldbe less than 20%. Protrusions, however, will make contact with the sheetbefore the remainder of the weld face. This can dramatically alter theinitial current density during the welding process. Intentionalprogramming of low initial current levels may be required for somegeometries to prevent overheating of the protruding electrode features.Once the protrusions have heated the sheet locally and penetrated thesurface, normal welding currents can be used. The amount of areamodified by the protrusions should be similar to that for intrusions, atmost 50% of the initial contact area and preferably less than 20%.

In order to produce a visible feature in the welded surface theprotrusions may require a minimum height of about ten microns above theface of the electrode. Similarly, intrusions in the face of an electrodemay require a minimum depth of about ten microns. These height and depthdimensions may be larger depending on the thicknesses of the workpiecesas described above.

The formation of attractive weld surfaces must not compromise structuralweld integrity. Structural weld quality is determined by the weldingparameters of force, time, and current, but is also closely linked tothe geometry of the electrode weld face. Electrode weld faces aretypically either flat or convex for welding of both steel and aluminum.For steel welding the electrode weld face should be a minimum of 10%greater than the target weld size which is 4×(t)^(1/2) where t is thegoverning metal thickness, i.e., the thinner sheet in a two-stack weld.The weld face can be as large as the diameter of the electrode, whichtypically varies from about 12 mm to about 20 mm. Curvature of the weldface can vary dramatically from flat to 6 mm in radius. Smooth-facedelectrodes are typically used for steel spot welding. For aluminum spotwelding the electrode weld face should be a minimum of 20% greater thanthe target weld size of 4×(t)^(1/2) where t is the governing metalthickness. The weld face can be as large as the diameter of theelectrode, which varies from about 16 mm to about 19 mm. Curvature ofthe weld face is more limited. It can vary from flat to a 50 mm radiusof curvature for smooth-faced electrodes, i.e., roughness of less than 1micron. For textured electrodes with a surface roughness greater than 1micron and preferably greater than 5 microns the radius of curvature canvary from flat to 20 mm.

For spot welding of both aluminum alloys and steel materials, electrodeswith flat weld faces leave indentations on the surface of the sheet thathave a flat central region. If this region is not aligned carefully withthe rest of the sheet then it appears off-angle and can degradeperceived quality. To prevent this from occurring, electrodes withradiused weld faces are preferred for producing high perceived qualitywelds. These would produce imprints with an overall dish-shapedappearance in the material.

An example of high perceived quality welds formed in zinc coated steelsheet are shown in FIG. 5. In this example the mirror images of theletters “GM” were formed as intrusions on the face of a weldingelectrode and used in forming a sequence of two spot welds.

FIG. 6 illustrates a weld electrode face 622 with protrusions 618 in theshape of a snow flake or star.

Practices of the invention have been disclosed in term of someillustrative embodiments which are not intended to limit the scope ofthe invention.

The invention claimed is:
 1. A welding electrode comprising: a body ofthe welding electrode, which welding electrode forms electricalresistance spot welds in an assembly of overlying metal workpieces thatcomprises at least one aluminum alloy workpiece, the body having an end;a convex weld face supported at the end of the body; and ringedprotrusions that project outwardly from the convex weld face and arepositioned to make contact with and indent into a surface of thealuminum alloy workpiece when the convex weld face of the weldingelectrode is brought into contact with the assembly of overlying sheetmetal workpieces during a resistance spot welding operation, the ringedprotrusions being radially spaced from one another on the convex weldface and each of the ringed protrusions having a circumference thatsurrounds a center of the convex weld face, wherein each of the ringedprotrusions has rounded corners when viewed in cross-section, andwherein an innermost ringed protrusion of the ringed protrusions risesto a height above a portion of the convex weld face surrounded by theinnermost ringed protrusion.
 2. The welding electrode set forth in claim1, wherein the convex weld face is spherically rounded and has a radiusof curvature between 20 mm and 50 mm.
 3. The welding electrode set forthin claim 1, wherein the welding electrode is formed of a copper alloy.4. The welding electrode set forth in claim 1, wherein an amount of areaof the convex weld face modified by the ringed protrusions is 50% orless of a contacting area of the convex weld face.
 5. The weldingelectrode set forth in claim 1, wherein each of the ringed protrusionsprotrudes above the convex weld face to a height of at least 10 μm. 6.The welding electrode set forth in claim 2, wherein the ringedprotrusions concentrically surround the center of the convex weld faceand are separated by concentric circular sloped portions of the convexweld face.
 7. The welding electrode set forth in claim 4, wherein theamount of area of the convex weld face modified by the ringedprotrusions is 20% or less of a contacting area of the convex weld face.8. A welding electrode comprising: a body of the welding electrode,which welding electrode forms electrical resistance spot welds in anassembly of overlying metal workpieces that comprises at least onealuminum alloy workpiece, the body having an end; a spherically roundedweld face supported at the end of the body, the spherically rounded weldface having a radius of curvature between 20 mm and 50 mm; and ringedprotrusions that project outwardly from the spherically rounded weldface and are positioned to make contact with and indent into a surfaceof the aluminum alloy workpiece when the spherically rounded weld faceof the welding electrode is brought into contact with the assembly ofoverlying sheet metal workpieces during a resistance spot weldingoperation, the ringed protrusions being radially spaced from one anotheron the spherically rounded weld face, wherein each of the ringedprotrusions having a circumference that surrounds a center of thespherically rounded weld face, and wherein an innermost ringedprotrusion of the ringed protrusions rises to a height above a portionof the spherically rounded weld face surrounded by the innermost ringedprotrusion.
 9. The welding electrode set forth in claim 8, wherein eachof the ringed protrusions has rounded corners when viewed incross-section.
 10. The welding electrode set forth in claim 8, whereinthe welding electrode is formed of a copper alloy.
 11. The weldingelectrode set forth in claim 8, wherein an amount of area of thespherically rounded weld face modified by the ringed protrusions is 50%or less of a contacting area of the spherically rounded weld face. 12.The welding electrode set forth in claim 8, wherein each of the ringedprotrusions protrudes above the spherically rounded weld face to aheight of at least 10 μm.
 13. The welding electrode set forth in claim8, wherein three or more ringed protrusions are included on thespherically rounded weld face.
 14. The welding electrode set forth inclaim 11, wherein the amount of area of the spherically rounded weldface modified by the ringed protrusions is 20% or less of a contactingarea of the spherically rounded weld face.
 15. A welding electrodecomprising: a body of the welding electrode, which welding electrodeforms electrical resistance spot welds in an assembly of overlying metalworkpieces that comprises at least one aluminum alloy workpiece, thebody having an end; a convex weld face supported at the end of the body;and ringed protrusions that project outwardly from the convex weld faceand are positioned to make contact with and indent into a surface of thealuminum alloy workpiece when the convex weld face of the weldingelectrode is brought into contact with the assembly of overlying sheetmetal workpieces during a resistance spot welding operation, the ringedprotrusions being radially separated from one another by portions of theconvex weld face and each of the ringed protrusions having acircumference that surrounds a center of the convex weld face, whereinan amount of area of the convex weld face modified by the ringedprotrusions is 50% or less of a contacting area of the convex weld face,and wherein an innermost ringed protrusion of the ringed protrusionsrises to a height above a portion of the convex weld face surrounded bythe innermost ringed protrusion.
 16. The welding electrode set forth inclaim 15, wherein each of the ringed protrusions has rounded cornerswhen viewed in cross-section.
 17. The welding electrode set forth inclaim 15, wherein the amount of area of the convex weld face modified bythe ringed protrusions is 20% or less of a contacting area of the convexweld face.
 18. The welding electrode set forth in claim 15, wherein theconvex weld face is spherically rounded and has a radius of curvaturebetween 20 mm and 50 mm.
 19. The welding electrode set forth in claim15, wherein each of the ringed protrusions protrude above the convexweld face to a height of at least 10 μm.
 20. The welding electrode setforth in claim 18, wherein the ringed protrusions concentricallysurround the center of the spherically rounded weld face and areseparated by concentric circular sloped portions of the sphericallyrounded weld face.